The present invention relates to an image recorder for recording an image by causing a laser beam to scan a photoconductive element.
A laser printer and a digital printer are typical examples of an image recorder of the type recording visible image or an electrostatic latent image formed on a photoconductive element by manipulating a laser beam having been modulated by pixel-by-pixel quantized image data. To form a multi-level image by manipulating a laser beam, use is often made of a pulse width modulation (PWM) system. An image recorder implemented by a PWM system is disclosed in Japanese Patent Laid-Open Publication (Kokai) No 74368/1988. The image recorder disclosed in this Laid-Open Publication has a laser writing control device for generating a multi-level output by PWM. The writing control device is capable of start writing image data from the right or from the left, as desired. When this type of image recorder is used to render tones by the combination of PWM and dither, a dot concentration type pattern whose tone gently varies is formed.
However, the prior art image recorder of the type described has a problem left unsolved, as follows. Assume that an image to be recorded has a halftone hairline which extends over two nearby pixels in a main scanning direction. Then, the harline is written by using two pulses each being modulated to a medium pulse width which is narrower than one pixel. As a result, despite that the original hairline I is a single line, it is rendered as two separate segements each being thinner than the original hairline. This kind of split is apt to occur not only with such a hairline but also with an image portion where a transition from low density to high density takes place, edges of a thick line, characters contitued by lines, figures, etc.
A PWM type laser beam printer which controls the writing pulse width in response to density information of an image signal having multi-level density information is also taught in Japanese Patent Publication No. 30792/1986 and Japanese Patent Laid-Open Publication No. 23535/1979.